Radiotelephones generally refer to communications terminals which provide a wireless communications link to one or more other communications terminals. Radiotelephones may be used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Many radiotelephones, particularly handheld radiotelephones, employ retractable antennas which may be extended out of, and retracted back into, the radiotelephone housing. Typically, retractable antennas are electrically connected to a printed circuit board located within the radiotelephone housing that contains radio frequency circuitry. The antenna and the radio frequency circuitry are typically interconnected such that the impedance of the antenna and the impedance of the radio frequency circuitry are substantially matched. Because radiotelephones use 50 ohm (.OMEGA.) impedance coaxial cable or microstrip transmission lines to connect the antenna to the radio frequency circuitry, such matching typically involves mechanically adjusting or electrically tuning the antenna so that it exhibits an impedance of approximately 50 ohms at its connection with the coaxial cable or microstrip transmission line.
Unfortunately, matching the impedance of a retractable antenna may be difficult because the antenna impedance may be dependent on the position of the antenna with respect to both the housing of the radiotelephone and the printed circuit board which contains the radio frequency circuitry. As these respective positions change when the antenna is moved between the extended and retracted positions, an antenna typically exhibits at least two different impedance states, both of which should be matched to the 50 ohm impedance of the feed from the printed circuit board. Accordingly, with retractable antennas, it is generally desirable to provide an impedance matching system that provides an acceptable impedance match between the antenna and the radio frequency circuitry, both when the antenna is retracted, and when the antenna is extended.
"Dual-band" radiotelephones transmit and receive signals in two or more separated frequency bands. Exemplary dual-band radiotelephones are those used with various satellite communications systems that employ widely separated transmit and receive frequency bands (e.g., 800 MHz and 1900 MHz). High performance 800 MHz radiotelephone antennas often take the form of a top loaded half-wave monopole. A helical top loading section may be used to mechanically shorten the antenna structure while maintaining the performance of a half-wave antenna. In the retracted position, the helical top loading section performs as a quarter-wave helical antenna. Dual-band performance may be achieved by either using a parasitic element in the helical top loading section that is resonant at 1900 MHz, or by inducing a secondary resonance in the helical top loading section at 1900 MHz.
Unfortunately, it may be difficult to deliver sufficient power to resonate the parasitic element or the helical top loading section at 1900 MHz when the antenna is in an extended position. As a result, performance approaching a half-wave monopole at 1900 MHz may be difficult to achieve. Performance may often be better when the antenna is in a retracted position. Furthermore, severe constraints may be placed on a matching network to achieve the band width and power transfer necessary for satisfactory dual-band radiotelephone performance.